Had the call with Eaton support today. Very nice guy that called me exactly on schedule. We walked through the issue and he recommended going to through the menu on the UPS to change Settings -> Battery Settings -> Battery Charge Mode from ABM Cycling to Constant Charge. His theory was that it could be doing the ABM cycling at the point I was doing my "unplug the input" test, and thus the battery was already low enough to trigger my under 80% and under 10% shutdown sequences. I was skeptical, since the battery indicated it was at 100% charge and the shutdown sequence start was triggered within 30 seconds of unplugging the main input. He said that the battery level indication isn't how much charge the battery has, but how much power is going to the battery. That would seem surprising to me though. Monitoring indicates the battery is at 99% when after unplugging the main and the shutdown sequence starts. He advised to try the test again ~5 hours after changing this Battery Charge Mode setting. 5 hours later, I tried the test, and had the same result with the shutdown sequence being initiated immediately upon unplugging the input main.

Gonna reach out to them to figure out the next steps.